The present invention relates to a gypsum panel product and process for making it. More particularly, the present invention relates to a gypsum panel with improved fire resistance using an expandable surface layer.
Gypsum panels are well known building products which have been used for years. They are used primarily as an interior wall and ceiling product, but also to a certain extent as an exterior product. A slurry including calcium sulfate hemihydrate and water is used to form the core, and continuously deposited on a paper cover sheet moving beneath a mixer. A second paper cover sheet is applied thereover and the resultant assembly is formed into the shape of a panel. Calcium sulfate hemihydrate or stucco reacts with sufficient water to convert the hemihydrate stucco into a matrix of interlocking calcium sulfate dihydrate crystals, causing it to set and to become firm. The continuous strip thus formed is conveyed on a belt until the calcined gypsum is set, and the strip is thereafter cut to form boards of desired length, which boards are conveyed through a drying kiln to remove excess moisture.
The chemically combined water in the dihydrate crystal contributes to the fire-retardant properties of the gypsum panel. When exposed to heat or fire, the dihydrate is converted back into the hemihydrate or even the anhydrite forms. Excess water of hydration is driven off in the form of steam. As the steam is released, heat transmission through the panel is reduced as the heat energy from the fire is used to drive the dehydration reactions and to vaporize the water. Thicker gypsum panels provide more fire resistance than thinner ones as it takes longer for the heat to penetrate the entire thickness of the panel to drive off the all crystal water.
The loss of the crystal water normally leads to shrinkage of the gypsum. Because much of the crystal water is lost and the original tight interlocking gypsum matrix is loosed, the gypsum products also tend to become very brittle and lost its strength and integrity under the fire. Large cracks can form in the gypsum panel due to the shrinkage. Shrinkage also causes enlargement of the opening between the panels at the edge joints. Enlargement of the edge joint, large cracks and other openings allow accelerated transmission of heat and hot gasses through the wall, and can also allow the fire to reach wood studs behind the gypsum panels to further fuel the fire.
Introduction of certain fibers, ores and/or inorganic particles into the gypsum core are known to improve the fire resistant properties of gypsum panels. The use of glass fibers to maintain the panel strength and integrity is taught in U.S. Pat. No. 4,647,486. This patent also reveals the addition of calcium sulfate anhydrite 11 to the core slurry to reduce shrinkage.
The introduction of expandable material into the gypsum core of a panel is also used as a method of improving the fire resistance of these panels. As the board heats up in the presence of the fire, intumescent materials expand to at least partially take the place of the water being displaced. It would take much longer for the board to burn through or form large cracks compared to non-expanding gypsum core. However, the use of intumescent materials throughout a gypsum core is relatively expensive and causes health and environmental concern because large amounts of the organic additive are needed for uniform dispersion throughout the gypsum slurry.
A wall or floor/ceiling assembly fire test simply measures the time it takes for the system to reach the limiting criteria specified in Standard ASTM (the American Society for Testing and Materials) E119 Test Procedure. For a wall assembly, the limiting criteria is defined as passage of heat energy through the wall, exceeding of a prescribed temperature rise on the unexposed face of the wall, the ability of the wall to carry a superimposed design load during the fire (for load-bearing walls), or projection of water through the assembly. Per ASTM C36, a ⅝-inch (16 mm) thick Type X panel must provide no less than a one-hour fire resistance rating when applied in a single layer on each face of a load-bearing wood-stud wall when tested in accordance with ASTM E119. A ½-inch (12 mm) Type X panel must provide a 45-minute fire resistance rating on the same assembly.
A second type of fire-resistant gypsum panel, known as Type C (enhanced Type X), provides even better performance. In addition to glass fiber additives, Type C panels contain the special minerals that expand in the presence of heat, somewhat compensating for the panel shrinkage resulting from the dehydration and melting of the gypsum. This helps add stability to the gypsum core, significantly enhancing the panel's fire-resistant performance.